1. Field of the Invention
This invention concerns an image display medium capable of repetitive rewriting by using particles, and an image forming apparatus.
2. Description of the Related Art
As image display media capable of repetitive rewriting, display techniques such as twisting ball display (particle rotation display by plural separately colored particles), electrophoresis, magnetic phoresis, thermal rewritable medium and liquid crystals having memory properties have been proposed so far. The display techniques described above are excellent in the memory property of images but involve a problem that a surface cannot provide a white display face such as paper and the density contrast is low.
As a display technique using a toner for solving the problem described above, there has been proposed a display technique of sealing electroconductive colored toners and white particles between facing electrode substrates, injecting static charges to the conductive colored toners by way of a charge transportation layer disposed on the inner surface of the electrode substrate on the non-display side, in which the charge-injected conductive colored toners are moved by an electric field given between both of the electrode substrates to the electrode substrate on the display side situated facing the electrode substrate on the non-display side, and deposited to the inside of the electrode substrate on the display side to display images by the contrast between the conductive colored toners and the white particles (Japan Hardcopy' 99 Reports p 249-252). This display technique is excellent in that all the image display media are constituted with solid materials and white and black (color) display can be switched to 100% in principle. However, in the technique described above, conductive colored toners not in contact with the charge transportation layer disposed on the inner surface of the electrode of the non-display substrate and the conductive colored toners separated from other conductive colored toners are present and such conductive colored toners are not moved by the electric field since the charges are not injected and present at random between both of the electrode substrates, so that it results in lowering of the density contrast.
Japanese Published Unexamined Patent Application No. 2000-98803 discloses an image display medium including a voltage application unit such as a charging roller, a magnetic roller, an exposing unit and a photoreceptive support as an image display medium substrate, in which two different kinds of fine particles including magnetic toners and non-magnetic toners are disposed between the substrates. In this medium, the magnetic toners are moved in accordance with the charges applied by the charging roller, then injected with charges from the photoreceptive support only at an exposure portion, to be neutralized with charges and loss possessed charges and then retracted by the magnetic roller. Image display is conducted by the contrast of the relative movement by the contrast of movement between the magnetic toners and the non-magnetic toner. The magnetic toners in this system are charge injection type toners with a lower electric resistance value. This system requires an on-demand image injection mechanism upon image display, which complicates the image forming mechanisms. Further, although the publication describes that the toners include a charge controller, it does not suggest the addition of a charge controller in insulative toners charged frictionally to each other.
The present inventors have proposed an image display device including a pair of substrates, plural kinds of particle groups sealed movably between the substrates by an applied electric field between the substrates and different in the color and the charging characteristics (Japanese Patent Application No. 2000-165138). According to this proposal, high whiteness and density contrast are obtained. The constitution of the particles in this proposal is excellent in the white density, the black density and the density contrast at the initial stage but, when conducting repetitive writing over a long time, the image density is sometimes lowered to lower the density contrast or the image uniformity is lowered to cause unevenness in the images.